Compact Planar Tunable Filter With Constant Absolute Bandwidth and Wide-Frequency Tuning Range Using DGS Coupling Structure

In this study, defected ground structures (DGSs) are applied in the coupling schemes of a planar tunable filter to achieve constant absolute bandwidth (CAB). The filter consists of parallel-coupled quarter-wavelength resonators loaded with varactor diodes, and the DGSs are implemented to enhance the coupling at a certain part. This method enables controlling the ratio between electric coupling and magnetic coupling and makes the total coupling coefficient inversely proportional to frequency, as required to achieve CAB. With the DGSs, the tunable resonators can be equalized to stepped-impedance tunable resonators, which bring wider-frequency tuning range. Moreover, due to the help of the DGS in adjusting the coupling coefficient curve versus frequency, the resonators maintain their universal widths and are coupled at full length. Thus, the tunable filter is as compact as a traditional combline filter. Meanwhile, each external coupling scheme contains a series capacitor and a short-ended stub that is parallel coupled to the first or last resonator, resulting in external
$Q$
factor being proportional to frequency. A second-order tunable filter is designed and measured, and a wide tuning range from 0.63 GHz to 1.09 GHz together with a constant bandwidth of 65±4 MHz are achieved. Besides, a transmission zero generated by coupling zero of the DGS-loaded coupling structure can be observed at the high stopband. The total dimension of the filter is 0.05
$\lambda _{\mathrm {g}}^{\ast }~0.15 \lambda _{\mathrm {g}}$
, which is more compact than solutions with partial-length coupling schemes. Owing to the simple structures of the resonators and coupling schemes, the filter solution has the potential to design high-order tunable filters with CAB, and a compact fourth-order tunable filter with ABC is given with simulation results.